The present invention is occupied with the problem to convert direct voltage into three-phase alternating voltage and conversely in all types of applications, such as in plants for transmission of electric power and for connecting three-phase electric machines to DC voltage loads and sources. Any ranges of voltages, currents and powers are conceivable.
The invention relates more particularly to such an apparatus comprising a VSC-converter (VSC=Voltage Source Converter) having a direct voltage intermediate link with a positive and a negative pole and at least one phase leg interconnecting the two poles and having at least two current valves connected in series, each current valve having at least one semiconductor device of turn-off type and a rectifying member connected in antiparallel therewith, the apparatus further comprising a transformer with two opposite ends of a first winding thereof connected to an output each of the VSC-converter and with a second winding thereof connected to an arrangement adapted to form voltage pulses for forming an alternating phase voltage, the apparatus also comprising a unit adapted to control the VSC-converter and said arrangement for obtaining said voltage conversion, and a method for converting direct voltage into alternating voltage and conversely according to the preamble of the appended independent method claim.
It is pointed out that xe2x80x9cfirst windingxe2x80x9d and xe2x80x9csecond windingxe2x80x9d are to be interpreted as a primary and a secondary winding of a transformer used for voltage transformation, although it is here not indicated which one is which.
xe2x80x9cRectifying memberxe2x80x9d is here and in the entire disclosure, including the appended claims, to be interpreted broadly, and it may be any member with ability to take a voltage and block current in at least one direction therethrough, and it does not have to be a diode, but it could for example also be controllable, such as a thyristor (see for example FIG. 3 of this disclosure). Furthermore, the rectifying member and the semiconductor device may also be integrated in one single semiconductor device or switching device. This means for the VSC-converter a semiconductor device with reverse conducting property, such as a MOSFET with an inherent xe2x80x9cbody diodexe2x80x9d.
An apparatus of this type may be used for converting direct voltage into alternating voltage and conversely in applications where it is important to obtain a galvanic isolation between the direct voltage side and the alternating voltage side. Furthermore, it is possible to obtain a voltage with variable frequency and amplitude on the alternating voltage side, a bilateral power flow and voltage as well as current transformation by an apparatus of this type.
A known such apparatus comprises a transformer that operates at the alternating voltage side frequency, which generally means a low frequency and thereby a heavy and bulky transformer. This results in a considerably lower efficiency of the transformer and thereby of the apparatus than would the transformer be able to operate at higher frequencies.
An apparatus of this type enabling operation of the transformer at higher frequencies than the alternating voltage frequency is known through xe2x80x9cPower loss reduction techniques for three-phase high frequency link DC-AC converterxe2x80x9d 24th Annual IEEE Power Electronics Specialists Conference, PESC ""93 Record. pp. 663-668, 1993 by I. Yamato and N. Tokunaga, and through xe2x80x9cHigh-frequency link DC/AC converter with suppressed voltage clamp circuits-naturally commutated phase angle control with self turnoff devicesxe2x80x9d, IEEE Transactions on Industry Applications, vol. 32, No. 2, pp. 293-300, March-April 1996 by M. Matsui, M. Nagai, M. Mochizuki and A. Nabae, the latter being shown in the appended FIG. 1. The reference numerals used there are as follows: direct voltage intermediate link 1, voltage source converter 2, transformer 3, arrangement 4, direct voltage side 5 and alternating voltage side 6. The arrangement on the alternating voltage side of the transformer is here a cycloconverter operating with natural commutation and converting the high frequency alternating voltage from the voltage source converter into an alternating voltage of the desired frequency. However, the voltage source converter still operates with forced commutation and hard switching resulting in comparatively high stresses on the semiconductor devices of the current valves resulting in comparatively high switching losses. Furthermore, the current valves of the voltage source converter are in the apparatus according to the latter reference controlled by a control unit 7 according to a method resulting in square voltage pulses with no zero-voltage interval thereby indirectly increasing the content of harmonics in the alternating voltage side output voltage. Besides the fact that the power lost in the form of heat results in considerable costs the semiconductor devices of the current valves have to either be dimensioned to be able to withstand high thermal stresses and thereby be costly or a lower frequency of the VSC-converter has to be applied resulting in a more bulky transformer and a degraded curve shape for the alternating current curves.
It is also known to utilize so called soft switching for reducing switching losses in apparatuses for converting direct voltage into alternating voltage and conversely, and these concepts generally incorporate additional semiconductor devices that do not take part in the power conversion itself. These additional (auxiliary) semiconductor devices and the control circuitry associated therewith add to the costs and complexity of such an apparatus. Moreover, they often involve a derating of the main semiconductor devices in the current valves either in terms of the maximum current or voltage.
The object of the present invention is to provide an apparatus of the type defined in the introduction having improved properties with respect to such apparatuses already known.
This object is according to the invention obtained by providing such an apparatus, in which the VSC-converter comprises at least one snubber capacitor connected to said current valves thereof, in which said arrangement comprises a direct converter having at least three phase legs connected through the opposite ends thereof to opposite ends of said second winding of the transformer and having at least two current valves connected in series, each of these current valves being able to conduct current and block voltage in both directions and to turn on by gate control, and in which the midpoints of said phase legs of the direct converter are provided with phase outputs for forming a terminal for said alternating phase voltage between these phase outputs.
The use of at least one such snubber capacitor in an apparatus of this type including a VSC-converter, a transformer and a direct converter results in a possibility to obtain soft switching of the semiconductor devices in the VSC-converter. This capacitor/these capacitors will be used as energy storing means and be discharged and recharged when changing the switching state of the VSC-converter remarkably reducing the voltage derivatives when the valves are switched and the direct converter commutating the current gives rise to further advantages with respect to switching losses and stresses for the semiconductor devices and rectifying members, and the former may also be turned on at zero-voltage and low current derivatives. The rectifying members, e.g. diodes, may be turned on at low voltage derivatives and turned off at zero-voltage and at low current derivatives. In the direct converter no hard turn-off capability is needed, but the valves may very well turn off at a current zero-crossing similar to the turn-off process in a conventional thyristor converter. Accordingly, the losses may be reduced in an apparatus of this type with respect to such apparatuses already known and thereby costs be saved. Less costly semiconductor devices may also be used thanks to the reduced thermal stresses thereon. The basic functionality of an apparatus of this type in the form of voltage conversion with variable frequency on the alternating voltage side, the bilateral power flow, galvanic isolation by a magnetic transformer and voltage and current transformation may of course still be obtained. Furthermore, this design of the apparatus enables a variety of different control regimes for adapting the operation of the apparatus to the conditions prevailing. The basic principle of the operation of the apparatus is that the switching state of the VSC-converter determines the sign of the voltage across the transformer and the switching state of the direct converter determines the direction of the current through the transformer. Fundamentally, it is necessary to commutate the VSC-converter, i.e. changing the sign of the transformer voltage, for being able to commutate the direct converter, which is necessary for being able to commutate the VSC-converter again and so on. The VSC-converter also has to be commutated on a regular basis for limiting the transformer flux, whereas the direct converter is modulated for obtaining an alternating voltage pulse pattern on said terminal. xe2x80x9cDirect converterxe2x80x9d is here defined as a converter having no energy storing means, such as a direct voltage intermediate link.
According to a preferred embodiment of the invention said current valves of the VSC-converter each comprises a said snubber capacitor connected in parallel with said semiconductor device and rectifying member. An alternative to provide the function of said at least one snubber capacitor is provided by the fact that according to another preferred embodiment the VSC-converter comprises one said snubber capacitor connected in parallel to said first transformer winding.
According to a preferred embodiment of the invention said VSC-converter has two said phase legs and said outputs connected to the ends of said first transformer winding are formed by a midpoint between current valves of a phase leg each, and according to another preferred embodiment of the invention the VSC-converter has one said phase leg, one of said outputs connected to the ends of said first transformer winding is formed by a midpoint between current valves of said phase leg, and the output connected to the opposite end of the first transformer winding is formed by a midpoint of the direct voltage intermediate link separated from both said positive and negative poles by at least one capacitor. The embodiment with a VSC-converter having two phase legs has the advantage of making it possible to obtain zero-voltage intervals across said first transformer winding. However, the embodiment with only one phase leg has the advantage of a smaller number of components with respect to the two-phase legs design.
According to a preferred embodiment of the invention, which has already been indicated above, the valves of the direct converter comprise a semiconductor device adapted to be turned off and thereby turn off the valve by zero-crossing of the current through the semiconductor devices resulting in soft switching properties.
According to another preferred embodiment of the invention the rectifying members of the valves of the direct converter are based on a material having a wide energy gap between the valence band and the conduction band, i.e. a band gap exceeding 2 eV, and are preferably of silicon carbide. Especially when the switching devices turn off at current zero-crossing the reverse recovery of the diodes may cause overvoltages across the valves and increased switching losses if traditional silicon diodes are used. However, this problem is solved by using diodes of such a material, especially of silicon carbide, which exhibit nearly ideal behaviour in terms of reverse recovery.
According to another preferred embodiment of the invention said control unit is adapted to control the semiconductor devices of the VSC-converter for changing the switching state of this converter, by changing the connection of at least one of said outputs thereof from one pole of said direct voltage intermediate link to the other while charging and discharging said snubber capacitor(xe2x88x92s) for lowering the voltage derivatives during turn-off of a semiconductor device. In an embodiment, in which the VSC-converter has two said phase legs, said control unit is adapted to commutate one phase leg of the VSC-converter at a time starting from a state in which the two midpoints are connected to different poles of the direct voltage intermediate link for obtaining an intermediate state in which said midpoints are connected to the same pole for applying a zero-voltage to the first winding of the transformer. Accordingly, this way of changing the switching state of the VSC-converter makes it possible to obtain zero-voltage intervals also at said alternating phase voltage terminal. Moreover, according to another preferred embodiment of the invention said control unit is adapted to varying in an appropriate fashion the order in which the phase legs of the VSC-converter are commutated, which results in a possibility to distribute the losses in the diodes and semiconductor switches equally over several switching cycles.
According to another preferred embodiment of the invention said control unit is adapted to control the semiconductor devices of the current valves of the VSC-converter for commutating both phase legs at the same time starting from the state in which the two midpoints are connected to different poles of the direct voltage intermediate link through a conducting semiconductor device each by turning these semiconductor devices of both said valves off. This control regime has the advantage of being somewhat simpler than the regime for commutating one phase leg at the time.
According to another preferred embodiment of the invention said control unit is adapted to commutate the phase legs of the direct converter when the power flow in the apparatus is directed from the alternating voltage side to the direct voltage side, i.e. from the direct converter to the VSC-converter, by controlling the current valves of those phase legs for changing the connection of the output thereof from one end of said second transformer winding to the other for changing the direction of the current through said second transformer winding enabling a change of the switching state of the VSC-converter. All phase legs of the direct converter have to be commutated in this way for changing the direction of the transformer current. A desired voltage pulse width modulation pattern may be achieved on the alternating phase voltage terminal by such a control.
According to another preferred embodiment of the invention constituting a further development of the embodiment just mentioned the control unit is, starting from a state in which the output of a phase leg of said direct converter is connected to a first end of the second transformer winding through a conducting first current valve, adapted to turn the other, second current valve of that phase leg on for short-circuiting the phase leg for opening a current path through the winding of the transformer in the direction of the voltage across the transformer, so that the second current valve gradually takes over the current through the transformer and the first current valve turns off by natural commutation as the current through it goes down to zero.
According to another preferred embodiment of the invention said control unit is adapted to control the conducting current valves of the phase legs of the VSC-converter to turn off for commutating the output of those phase legs and at the same time, starting from a state in which the output of a phase leg of the direct converter is connected to a first end of the second transformer winding through a conducting first current valve, control the other, second current valve of that phase leg to turn on for short-circuiting the second transformer winding through that phase leg for opening a current path through that winding of the transformer in the direction of the voltage across the transformer so as to form a resonance circuit by the capacitance of the snubber capacitor(xe2x88x92s) of the VSC-converter and the leakage inductance of the transformer making the current through said first transformer winding increasing for assisting the commutation of said phase legs of the VSC-converter by charging and discharging said snubber capacitor(xe2x88x92s). This embodiment takes care of a problem that may be severe under certain conditions, namely when the current on the alternating voltage side of the apparatus is low, since it may then be impossible to commutate the VSC-converter in the normal way. The current through the transformer may then be insufficient for recharging the snubber capacitor(xe2x88x92s) regardless of the switch state of the direct converter. The recharge of the snubber capacitors may take too long time or in the extreme case when iAC,i=0, i=1, 2, 3 will not occur at all. By forming the resonance circuit in this way a resonance process governed by the snubber capacitances and the leakage inductance is initiated. Through this process the snubber capacitor(xe2x88x92s) are recharged so that the potential of the phase outputs of the phase legs of the VSC-converter swing to the opposite pole of the direct voltage intermediate link. This also means that the transformer voltage changes direction.
According to another preferred embodiment of the invention the apparatus comprises an additional inductor connected in series with said first transformer winding for increasing the inductance of said resonance circuit. This means that the time required for changing the switching state of the VSC-converter may be prolonged.
According to another preferred embodiment of the invention said control unit is adapted to a) control the semiconductor devices of the VSC-converter for changing the switching state of this converter by changing the connection of at least one of said outputs thereof from one pole of said direct voltage intermediate link to the other for changing the sign of the voltage across said first transformer winding and b) commutate the phase legs of the direct converter for changing the end of the second transformer winding to which the respective phase output is connected in such a sequence and at such delays that desired voltage pulses are obtained on said terminal and do this until the current through the second transformer winding has changed direction, and then start over with controlling the VSC-converter to change switching state again. This is a preferred generic commutation strategy to be used, in which it is assumed that the power initially flows from the direct voltage side to the alternating voltage side, and in the opposite case it is started by step b) followed by step a) and then by step b) again.
According to another preferred embodiment of the invention the control unit is adapted to control the semiconductor devices of the VSC-converter for changing the switching state thereof and start commutating one or several phase legs of the direct converter by controlling a current valve of that (those) phase leg(s) before the change of switching state of the VSC-converter has been completed, when there is a desire to avoid the state the system appears in after a commutation of the VSC-converter. By using such an interlaced commutation of the two converters an interval of each switching cycle during which the power flow will be of the opposite direction with regard to the desired direction may be reduced and the overall commutation speed can be increased. The apparatus has then preferably means for detecting the voltage across said first transformer winding, and the control unit is adapted to start the commutation of the direct converter based upon information from said voltage detecting means when, as a consequence of the change of switching state of the VSC-converter commenced, the voltage across the first transformer winding has changed sign and exceeded a predetermined threshold voltage value. It is thereby ensured that a complete commutation of both converters takes place, since it is for that necessary that the polarity of the transformer voltage is reversed and have reached a certain magnitude before the commutation of the direct converter is initiated.
According to another preferred embodiment of the invention, also relating to interlaced commutation, the control unit is adapted to commutate all phase legs of the direct converter by controlling the current valves of the phase legs and start controlling the semiconductor devices of the VSC-converter for changing the switching state thereof before the commutation of all phase legs of the direct converter has been completed, when there is a desire to avoid the state the system appears in after a commutation of all of the phase legs of the direct converter. When a power flow in that direction is desired it is also preferred to provide the apparatus with means for detecting the current through the second transformer winding, and the control unit is adapted to start the control of the VSC-converter for changing the switching state thereof based upon information from said current detecting means when, as a consequence of the commutation of the phase legs of the direct converter commenced, the current through the second transformer winding has changed direction and exceeded a predetermined threshold current value. A complete commutation of both converters is ensured when the direction of the transformer current has been reversed and reached a certain magnitude before the commutation of the VSC-converter is initiated.
The invention also relates to a method for converting direct voltage into alternating voltage and conversely according to the independent method claim. The advantages of such methods and of methods according to preferred embodiments of the invention defined in the dependent method claims appear without any doubt from the above discussion of the apparatus according to the preferred embodiments of the invention.
The invention also relates to a computer program product and a computer readable medium according to the corresponding appended claims. It is easily understood that the method according to the invention defined in the appended said method claims is well suited to be carried out through program instructions from a processor adapted to be influenced by a computer program provided with the program steps in question.
Further advantages as well as advantageous features of the invention appear from the following description and the other dependent claims.